The coalition had no immediate comment on the report released Monday.

The report by Airwars, a project aimed at tracking the international airstrikes targeting the Islamic State group, says it counted at least 459 suspected civilian fatalities in 57 airstrikes it believes the coalition carried out in Iraq and Syria over the last year. It says the same strikes also caused at least 48 suspected "friendly fire" deaths.

The report says that efforts to limit the risk to civilians are hampered by an absence of effective transparency and accountability from nearly all coalition members.

So far, the U.S. only has acknowledged killing two civilians in its strikes.